A Passive Optical Network (PON) is a point-to-multipoint fibre network architecture, in which unpowered (i.e. passive) optical splitters are used to enable a single optical fibre to serve multiple premises. A PON comprises at least one Optical Line Terminal (OLT) placed at the central office of the service provider and a number of Optical Network Terminations (ONT) near end users of the PON. More rarely, the OLT may be placed in an outside plant, e.g. at a street cabinet. One of the benefits of PONs is that the amount of fibre and the central office equipment required, can be reduced, as compared to point-to-point architectures. Another benefit of PONs is an anticipated much lower cost for field maintenance and fault management, as compared to metallic media networks. These costs are often referred to as “operational expenditures”.
In order to achieve the lower operational expenditures, it is important to be able to easily detect any occurring faults in the PON. A fault detection or fault indication should enable the operator of the PON to ascertain if the fault occurs in the fibre network or in the end equipment, and what kind of fault it is (i.e. identification). It is also desirable to get an indication of what part of the fibre network and where in the fibre network the fault occurs (i.e. locating the fault).
In today's fault management solutions, Optical Time Domain Reflectometry (OTDR) is typically used. An OTDR device or OTDR instrument sends a short pulse down the fibre and detects the back reflection or back scattering from that fibre.
One important factor in optical fibre loss is the scattering of light.
The light is scattered in all directions in the fibre, including backwards, towards the source of light. The light that is scattered backwards towards the source of light, i.e. the back reflection or back scattering, is measured in the OTDR device or OTDR instrument. Depending on the condition of the fibre and the existence of any connectors and the like, the scattering varies as the light passes down the fibre, from having been sent out from the OTDR device/instrument. It is possible to calibrate the speed of the pulse as it passes down the fibre. By doing so, it is possible for the OTDR device/instrument to correlate what it “sees” in the back scattered or back reflected light with an actual location in the fibre. It is therefore possible to create a display of the amount of back scattered light at any point in the fibre.
By analysing the back scattered light, it is possible to detect any fault in the fibre, such as water intrusion, ageing, faulty connectors, splices, fibre bends, crunches on the fibre, and so on. Any such condition will affect the scattering of light.
The OTDR device or OTDR instrument is typically very expensive and therefore it is desirable to have as few OTDR devices as possible to perform the measurements and the analysis. It is also desirable to perform the measurements using the OTDR device in a centrally located office, not having to bring the device or instrument into the field. However, a PON comprises one or more power splitters, which are associated with high losses. Thus, the OTDR signal is weakened after double-passing these splitters making any localisation of a fault after such a splitter very difficult. The bigger the power splitter, the bigger the loss is associated with it. A PON typically comprises power splitters with a split ratio of 1:32 or higher, making the localisation of any fault following the splitter impossible. At best, it is possible to detect a fault, but impossible to locate it.
Also, an optical power measurement at the subscriber side may be employed to detect any changes in received power, but it will not be possible to find the cause of the fault nor the location of the fault.
The ONTs may also send status messages back to the central office, in order to detect faulty fibres, optical components or faulty ONTs. However, a status message will merely indicate that a fault may have occurred, but it will not be possible to find the cause of the fault nor the location of the fault.